The present invention relates to an electron beam focusing device for use in a cathode ray tube (CRT), and more particularly to an electron beam focusing device that can provide a large diameter electrostatic lens.
Electron beams in a CRT are inclined to spread immediately before the focusing point of a screen. This is called the divergence of beams. Therefor, in order to improve the focusing characteristic of a CRT, it is necessary to converge the electron beams with a larger deflection angle which can be provided by increasing the diameter of an electron lens formed by an electron gun.
To this end, it has been proposed to provide an electron lens using the inner diameter of a neck tube as disclosed, for example, in JP-A-57-192051 (U) filed on May 29, 1981. FIG. 2 shows a section of the conventional CRT including such an electron lens. As shown in FIG. 2, a first conductor film 3 and a second conductor film 4 spaced by a predetermined distance l are formed on the inner surface of the neck tube 2 of a CRT 1; the first conductor film 3 is connected with an anode button (not shown). An electron gun 10 is constituted by a cathode electrode 5, a first grid electrode (G.sub.1) 6, a second grid electrode (G.sub.2) 7, a third grid electrode (G.sub.3) 8, and a contact member 9 for contact. The second conductor film 4 is connected with the third grid electrode (G ) 8 through the contact member 9. The third grid electrode 8 is connected with a pin 13 of a stem 12 through a connector 11, and the pin 13 is connected with a focusing power supply 14.
A high voltage is applied to the first conductor film 3 through the anode button, whereas a predetermined voltage is applied to the second conductor film 4 from the focusing power supply 14 though the pin 13, the connector 11, the G.sub.3 electrode 8 and the conductor member 9. An electrostatic lens 101 is formed by the first conductor film 3 and the second conductor film 4 spaced apart from each other by a predetermined distance l. The diameter thereof is so large as to be substantially equal to the inner diameter of the neck tube 2.
The prior art mentioned above has the following two serious problems
The first problem is: since there is a great potential difference (about 20 KV) between the first conductor film 3 and the second conductor film 4, electric field concentration will occur at the ends of the films if the distance l is relatively short, thereby causing discharge.
The second problem is as follows. The surface potential at the inner surface of the neck tube 2 corresponding to the distance l, which is made of an insulator, is always unstable and not uniform due to dirt on the inner surface and the secondary electron multiplication phenomenon by "stray emission" at the insulator portion. Thus, the electric field of the electrostatic lens 101 formed by the first conductor film 3 and the second conductor film 4 is also not uniform, thereby providing undesirable astigmatism. Accordingly, beam spots having various forms other than an axis-symmetrical form will be provided. Also, the diameter of the beam spots can not be satisfactorily converged. Further, the non-uniform electric field mentioned above provides a secular change in the average locus of the electron beams so that image positions are also changed on the screen.
The above two problems are related with each other. Namely, although the blocking voltage capability or withstand voltage can be enhanced if the distance l is increased, the electric field instability will be further amplified, thereby making it impossible to actually use the electrostatic lens.